U.S. Pat. No. 4,867,337 discloses a threaded closure for a fuel tank. The threaded closure includes a cylindrical, cup-shaped base body having an internal thread. An annular wall disposed inwardly is formed on the base wall of the base body and stands perpendicularly to this base wall. The annular wall and the cylindrical wall conjointly delimit a receiving space in which a flat sealing ring is inserted without play. A limp connecting part is fixed to the base body within the annular wall and includes a retaining member which prevents the closure from becoming separated from the tank and becoming lost.
The sealing ring is made mostly of rubber and swells with time because of the contact with the fuel and is pressed out of the receiving space in the base body of the threaded closure. Since no further retention is provided, the danger is present that the sealing ring will become lost and a leak-proof closure of the fuel tank is no longer ensured.